Dusty shells surrounding the carbon variables S Scuti and RT Capricorni

TL;DR

This study uses Herschel PACS observations to analyze dusty shells around the carbon variables S Sct and RT Cap, revealing detailed shell structures and mass-loss characteristics of these evolved stars.

Contribution

It provides the first detailed imaging and modeling of detached shells around S Sct and RT Cap, enhancing understanding of mass-loss processes in carbon-rich AGB stars.

Findings

S Sct has a bright, spherical detached shell co-spatial with CO structures.

RT Cap has two shells, with the inner being spherical and the outer irregular.

Derived dust temperatures, shell masses, and mass-loss rates for both stars.

Abstract

For the Mass-loss of Evolved StarS (MESS) programme, the unprecedented spatial resolution of the PACS photometer on board the Herschel space observatory was employed to map the dusty environments of asymptotic giant branch (AGB) and red supergiant (RSG) stars. Among the morphologically heterogeneous sample, a small fraction of targets is enclosed by spherically symmetric detached envelopes. Based on observations in the 70 {\mu}m and 160 {\mu}m wavelength bands, we investigated the surroundings of the two carbon semiregular variables S Sct and RT Cap, which both show evidence for a history of highly variable mass-loss. S Sct exhibits a bright, spherically symmetric detached shell, 138" in diameter and co-spatial with an already known CO structure. Moreover, weak emission is detected at the outskirts, where the morphology seems indicative of a mild shaping by interaction of the wind with the interstellar medium, which is also supported by the stellar space motion. Two shells are found around RT Cap that were not known so far in either dust emission or from molecular line observations. The inner shell with a diameter of 188" shows an almost immaculate spherical symmetry, while the outer ~5' structure is more irregularly shaped. MoD, a modification of the DUSTY radiative transfer code, was used to model the detached shells. Dust temperatures, shell dust masses, and mass-loss rates are derived for both targets.